this issue says that osmo-msc should check that the cipher matches the request,and the issue implies that osmo-msc doesn't check that.If it turns out that osmo-msc does indeed reject a mismatch already, that's nice.

How about the side thing there, if the chosen algorithm is not provided in the response?I guess osmo-msc should accept then. Does it? (If they mismatch then, the ciphered data will not be decipherable anyway.)Maybe duplicate the test for that situation.